Switching mode power supply capable of achieving soft transition for valley switching, or performing valley locking

ABSTRACT

A quasi-resonant (QR) converter introduces soft transition for valley switching and valley locking. One exemplifying QR converter is capable of performing valley switching, but has the valley switching in a specific signal valley of one switching cycle softly transit to the valley switching in another signal valley of a later switching cycle, via at least one switching cycle performing no valley switching.

BACKGROUND

The present disclosure relates generally to switching mode power supplies.

Switching mode power supplies (SMPS) typically utilize a power switch to control the current through an inductive device in order to regulate a current output or a voltage output. In comparison with other kinds of power supplies, SMPS are commonly compact and power efficient, so as being popular nowadays.

One kind of SMPS operates in quasi-resonance (QR) mode and is referred to QR converters. The power switch in a QR converter is switched from an OFF state (performing an OFF circuit) to an ON state (performing a short circuit) substantially at the moment when the voltage drop across the power switch is at a minimum, so the switching loss might be minimized, theoretically. Observation has proofed that the power conversion efficiency of a QR converter is really excellent especially when it supplies power to a heavy load.

FIG. 1 demonstrates a QR converter 10 in the art, where a transformer, an inductive device, has a primary winding PRM, a secondary winding SEC and an auxiliary winding AUX, all inductively coupled to each other. The QR converter 10 is powered by input voltage V_(IN), to supply power, in the form of output voltage V_(OUT) and output current I_(OUT), to an output load 24. QR converter 10 provides pulse-width-modulation (PWM) signal V_(GATE) at driving node GATE to periodically turn ON and OFF a power switch 34. Via the voltage division provided from resistors 28 and 30, QR converter 10 further monitors voltage drop V_(AUX) across the auxiliary winding AUX. FIG. 2 illustrates the waveforms of PWM signal V_(GATE) and voltage drop V_(AUX). Shown in FIG. 2, two consecutive rising edges of PWM signal V_(GATE) define one switching cycle, whose duration is referred to as a cycle time T_(CYC) consisting of an ON time T_(ON) and an OFF time T_(OFF), where the ON time T_(ON) and the OFF time T_(OFF) are the durations when the power switch 34 is kept as being ON and OFF, respectively. FIG. 2 also demonstrates that the ON time T_(ON) is also the pulse width of the PWM signal V_(GATE). Demonstrated in FIG. 2, about the middle of the OFF time T_(OFF), the voltage drop V_(AUX) starts oscillating because of the power depletion of the transformer, and signal valleys VL₁ and VL₂ are therefore generated. QR controller 26 ends a cycle time T_(CYC) or an OFF time T_(OFF) at the moment when signal valley VL₂ substantially occurs as demonstrated in FIG. 2. This kind of method to end a cycle time T_(CYC) in a signal valley is known and referred to as valley switching.

QR converter 10 has, at a compensation node COMP, a compensation signal V_(COMP), controlled by operational amplifier (OP) 20, in response to the difference between the output voltage V_(OUT) and a target voltage V_(TAR). The compensation signal V_(COMP) in the QR converter 10 controls both the ON time T_(ON) and a block time T_(BLOCK), where the next switching cycle is not allowed to start until the block time T_(BLOCK) ends. The block time T_(BLOCK) prevents a switching frequency f_(CYC), the reciprocal of a cycle time T_(CYC), from being over high. An over-high switching frequency f_(CYC) probably lowers the power conversion due to the more power loss in charging and discharging the driving node GATE. The block time T_(BLOCK) equivalently defines a maximum switching frequency f_(CYC-MAX), which is the reciprocal of the block time T_(BLOCK).

QR converter 10 usually encounters two issues.

The first issue is the hardship to solve electromagnetic interference (EMI). For a constant output load 24, the compensation signal V_(COMP) could be a constant, and the power switch 34 is turned on in a certain signal valley to conclude a cycle time T_(CYC), implying a constant switching frequency f_(CYC) and intensive EMI, normally unacceptable in the art. A known solution for this EMI issue is to intentionally disturb the compensation signal V_(COMP). The feedback loop provided by the operational amplifier 20 in FIG. 1, however, tends to cancel any disturbance introduced to the compensation signal V_(COMP). Therefore, this solution hardly helps the EMI issue.

Another issue is the occurrence of intolerable audible noise. In some conditions with a certain output load 24, the compensation signal V_(COMP) spontaneously vibrates, and QR controller 26 performs valley switching not constantly in a certain signal valley, but alternatively in two adjacent signal valleys. In other words, due to the vibration of the compensation signal V_(COMP), valley switching might be first in a certain signal valley for several switching cycles, then followed by shifting to be in an adjacent signal valley for a while, and then further followed by shifting back to be in the certain signal valley for a while, and so forth. This instability in valley switching could result in audible noise, which is normally intolerable in the market, especially for the applications targeting to a quiet environment.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted.

The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 demonstrates a QR converter in the art;

FIG. 2 illustrates the waveforms of PWM signal V_(GATE) and voltage drop V_(AUX);

FIG. 3 shows a QR controller, which in some embodiments of the invention replaces the QR controller in FIG. 1;

FIG. 4 demonstrates some signals in FIG. 1 when the QR controller 26 is replaced by the QR controller of FIG. 3;

FIG. 5 exemplifies a current estimator;

FIG. 6 demonstrates a one-to-one relationship between the load representative signal V_(L-EST) and the output current I_(OUT);

FIG. 7 demonstrates a possible relationship between the maximum switching frequency f_(CYC-MAX) and the output current I_(OUT);

FIG. 8 shows a power controller, which in some embodiments of the invention replaces the QR controller 26 in FIG. 1;

FIG. 9 shows a QR controller, which in an embodiment of the invention is a replacement for the QR controller 26 in FIG. 1;

FIG. 10 demonstrates some waveforms of signals in FIG. 1 when the QR controller 26 is replaced by the QR controller 300;

FIG. 11 shows a control method adapted by the OFF time controller;

FIG. 12 shows the waveforms of the drop voltage V_(AUX) and several signal timings during several consecutive switching cycles when the output load turns from heavy into light;

FIG. 13 shows the waveforms of the drop voltage V_(AUX) and several signal timings during several consecutive switching cycles when an output load turns from light into heavy;

FIG. 14 shows possible variation to the oscillation time V_(S-VL) of the prior art; and

FIG. 15 shows possible variation to the oscillation time T_(S-VL) according to one embodiment of the invention.

DETAILED DESCRIPTION

In one embodiment of the invention, a compensation signal V_(COMP) of a power controller controls the ON time T_(ON), but not the block time T_(BLOCK), which instead is in response to a load representative signal V_(L-EST) that is capable of representing a present output current I_(OUT) to an output load. The power controller detects an auxiliary winding AUX to determine a discharge time T_(DIS) of the transformer. The load representative signal V_(L-EST) could be derived by the discharge time T_(DIS) and a current sense signal V_(CS), which indicates the current passing through the primary winding PRM of the transformer. The block time T_(BLOCK) is in response to the load representative signal V_(L-EST). The power controller is not allowed ending a cycle time T_(CYC) until the block time T_(BLOCK) elapses.

Simply speaking, in one embodiment of the invention, the ON time T_(ON) and the block time T_(BLOCK) are in response to the compensation signal V_(COMP) and the load representative signal V_(L-EST), respectively.

Based on this design concept, if the output current I_(OUT) to an output load is a constant, the block time T_(BLOCK) will be about a corresponding constant accordingly. Meanwhile, the feedback loop provided by the operational amplifier 20 automatically adjusts the compensation signal V_(COMP) to provide an appropriate ON time T_(ON), for sustaining the output current I_(OUT). It can be concluded that valley switching could be performed at a certain signal valley for a constant output load and that the instability of valley switching in the prior art could be eliminated.

One embodiment of the invention jitters the block time T_(BLOCK), in order to solve the possible EMI issue due to the valley switching in a certain signal valley for a constant output load. Jittering to the block time T_(BLOCK) certainly influences the compensation signal V_(COMP), which in one embodiment of this invention has no impact to the block time T_(BLOCK), because the block time T_(BLOCK) is substantially in response only to the jittering and the output current I_(OUT) while the output current I_(OUT) is a constant during the measurement of EMI. Unlike what happens in the prior art, the jittering to the block time T_(BLOCK) will not be tapered by the feedback loop in the power converter. Therefore, jittering to the block time T_(BLOCK) could vary the block time T_(BLOCK) to a predetermined extent, so as to effectively jitter the switching frequency f_(CYC) and solve the EMI issue.

FIG. 3 shows a QR controller 80, which in some embodiments of the invention replaces the QR controller 26 in FIG. 1. Exemplified in FIG. 3, the QR controller 80 includes a valley detector 82, a discharge time detector 84, output current estimator 86, an And gate 88, a block time generator 90, a jittering apparatus 92, and a pulse width modulator (PWM) 94. FIG. 4 demonstrates some signals in FIG. 1 when the QR controller 26 is replaced by the QR controller 80 of FIG. 3. Please refer to FIGS. 1, 3, and 4 for the following description.

Via the detection node QRD and the voltage divider consisting of resistors 30 and 28, the discharge time detector 84 is coupled to the auxiliary winding AUX to generate a discharge time signal S_(TDIS) based on the voltage drop V_(AUX) across the auxiliary winding AUX. The discharge time signal S_(TDIS) is capable of indicating the duration of a discharge time T_(DIS) when the transformer continues de-energizing. For example, the waveform of the discharge time signal S_(TDIS) in FIG. 4 illustrates that a discharge time T_(DIS) starts at time point t₁ when the first rising edge of the voltage drop V_(AUX) happens in a switching cycle, and ends at time point t₂ when a following falling edge of the voltage drop V_(AUX) occurs.

The valley detector 82 also detects, via the detection node QRD, the voltage drop V_(AUX) to find signal valleys. The moment of generating a pulse to the valley indication signal S_(VD) virtually indicates the occurrence of a corresponding signal valley of the voltage drop V_(AUX). A common method employed in the valley detector 82 is to provide a pulse to the valley indication signal S_(VD) a predetermined delay time later once the voltage drop V_(AUX) drops across 0V. As demonstrated by the waveforms of the voltage drop V_(AUX) and the valley indication signal S_(VD) in FIG. 4, during an OFF time T_(OFF), the voltage drop V_(AUX) drops across 0V the first time at time point t₃, implying the beginning of the signal valley VL₁, then after a certain delay time at time point t₄ the valley indication signal S_(VD) has a pulse. Similarly, a certain delay time after the beginning of the signal valley VL₂, the valley indication signal S_(VD) has another pulse.

Demonstrated in FIG. 3, the output current estimator 86 receives the current sense signal V_(CS) and the discharge time signal S_(TDIS), and accordingly provides a load representative signal V_(L-EST). The current sense signal V_(CS) is at a current detection node CS and represents the current I_(CS) following through the resistor 36, which also represents the current I_(PRM) flowing through the primary winding PRM. The load representative signal V_(L-EST) could represent the output current I_(OUT) to the output load 24, even though it is just an estimative result based on the current sense signal V_(CS) and the discharge time signal S_(TDIS). The operation and theory used in the output current estimator 86 will be detailed later.

The block time generator 90 provides a block signal S_(BLOCK) to indicate a block time T_(BLOCK), in response to the load representative signal V_(L-EST). For example, the block time T_(BLOCK) is in positive correlation with the load representative signal V_(L-EST) or the larger load representative signal V_(L-EST) the larger block time T_(BLOCK). As shown by the waveform of the block signal S_(BLOCK) in FIG. 4, the cycle time T_(CYC) and the block time T_(BLOCK) substantially start at the same time (at time point t_(STR)), and the block time T_(BLOCK) concludes at time point t_(RELEASE). Hereinafter, the occurrence of the time point t_(RELEASE) means the conclusion of the block time T_(BLOCK).

The jittering apparatus 92 in FIG. 3, connected to the block time generator 90, provides a control signal S_(JITTER) to slightly and slowly alter the block time T_(BLOCK). For example, in a stable condition when the output load 24 is a constant, the control signal S_(JITTER) is a periodic signal with a jittering frequency of 400 Hz, and makes the block time T_(BLOCK) change in a range from 1/(27.5 kHz) to 1/(25 kHz), such that the switching frequency f_(CYC) could vary in a frequency range from 25 kHz to 27.5 kHz. Preferably, the jittering frequency of the control signal S_(JITTER), which is 400 Hz for example, is much smaller than the switching frequency f_(CYC) of the power converter, which is about tens of kilohertz in practice.

The And gate 88 has two inputs respectively coupled to the block time generator 90 and the valley detector 82, for transmitting the pulse in the valley indication signal S_(VD) to set the PWM 94 only after the block time T_(BLOCK) ends. Shown by the waveforms of the valley indication signal S_(VD) and the block signal S_(BLOCK) in FIG. 4, at the time point t_(END) after the block time T_(BLOCK) ends, a pulse is provided to the valley indication signal S_(VD), and this pulse passes through the And gate 88 to set the PWM 94, making the PWM signal V_(GATE) “1” in logic and concluding both the OFF time T_(OFF) and the cycle time T_(CYC). The And gate 88 ends a cycle time T_(CYC) substantially at the moment (t_(END)) when the first signal valley, which is the signal valley VL₃ in FIG. 4, occurs after the end of the block time T_(BLOCK). The time point t_(END) in one switching cycle is equivalent to the time point t_(STR) in the next switching cycle.

As demonstrated in FIG. 4, when the PWM signal V_(GATE) is set to be “1” in logic, the power switch 34 is turned on, and both the cycle time T_(CYC) and the ON time T_(ON) begin. The PWM 94 determines the duration of the ON time T_(ON) in response to the current sense signal V_(CS) and the compensation signal V_(COMP). For example, there in FIG. 4 is another compensation signal V_(COMP-SCALED), which is a scaled version of the compensation signal V_(COMP). Once the current sense signal V_(CS) exceeds the compensation signal V_(COMP-SCALED), the PWM signal V_(GATE) is reset to be “0” in logic, such that the ON time T_(ON) ends and the OFF time T_(OFF) begins.

FIG. 5 exemplifies the current estimator 86, which has a transconductor 190, a level shifter 192, an update circuit 196, an accumulative capacitor 198, a switch 104, a voltage-controlled current source 102, and a CS peak voltage detector 100.

The CS peak voltage detector 100 generates a signal V_(CS-PEAK) representing a peak of the current sense signal V_(CS). One example of the CS peak voltage detector 100 could be found in FIG. 10 of US patent application publication US20100321956, which is incorporated by reference in its entirety. Some embodiments of the invention might use the average current detector in FIG. 17 or 18 of US20100321956 to replace the CS peak voltage detector 100. The voltage-controlled current source 102 receives and converts the signal V_(CS-PEAK) into a discharge current I_(DIS), which drains from node ACC when the discharge time signal S_(TDIS) is “1” in logic. In other words, the total time that the discharge current I_(DIS) drains from the node ACC is about the discharge time T_(DIS). Some other embodiments could omit the switch 104 in FIG. 5, but use the discharge signal S_(TDIS) to enable or disable the voltage-controlled current source 102 instead. The voltage V_(M) on the capacitor 199 is level shifted to be the load representative signal V_(L-EST), which is compared with a predetermined reference voltage V_(REF) by transconductor 190. Based on the comparison result, the transconductor 190 outputs a charge current I_(CHARGE) to constantly charge the node ACC. The update circuit 196, capable of being triggered by signal S_(UPDATE), samples the voltage V_(ACC) at the node ACC to update the voltage V_(M). In one embodiment, the voltage V_(M) is updated once every switching cycle. For example, the signal S_(UPDATE) is equivalent to the PWM signal V_(GATE) in light of their logic values, implying the voltage V_(M) is updated every time when the OFF time just begins. Some embodiments might update voltage V_(M) once every several switching cycles, nevertheless. The voltage V_(M) is held as a constant, until it is updated to become another constant. As derivable from the teaching in this specification, the charge current I_(CHARGE) is a constant as long as the voltage V_(M) is kept as unchanged.

In one cycle time T_(CYC), the accumulative capacitor 198 accumulates the difference between an integral of the charge current I_(CHARGE) over a cycle time T_(CYC) and another integral of the discharge current I_(DIS) over the discharge time T_(DIS).

Similar to the analysis disclosed in US20100321956, if the voltage V_(ACC) at the moment when it is currently being sampled is the same as the voltage V_(ACC) at the moment when it was sampled last time, the charge current I_(CHARGE) is substantially in proportion to the output current I_(OUT) to the output load 24. In other words, the charge current I_(CHARGE) could represent the output current I_(OUT) if the sampled result of the voltage V_(ACC) has no influence to the voltage V_(M). The update circuit 196, the level shifter 192 a and the transconductor 190 together as a whole form a loop with a negative loop gain, to stabilize the voltage V_(ACC) at the moment when being sampled. For example, if the present charge current I_(CHARGE) is, to some extent, larger than a corresponding value representing the output current I_(OUT), the voltage V_(ACC) will become larger at the moment when sampled the next time, enlarging voltage V_(M) and decreasing the charge current I_(CHARGE), such that the charge current I_(CHARGE) approaches the corresponding value, and vice versa. With a proper negative loop gain, the voltage V_(M) can steadily approach to a constant over time, resulting in the charge current I_(CHARGE) in proportion to the output current I_(OUT).

FIG. 6 demonstrates a one-to-one relationship between the load representative signal V_(L-EST) and the output current I_(OUT). Accordingly, the load representative signal V_(L-EST) could represent the output current I_(OUT).

The load representative signal V_(L-EST) substantially determines a block time T_(BLOCK), such that the output current I_(OUT) substantially determines the block time T_(BLOCK) and the maximum switching frequency f_(CYC-MAX) (=1/T_(BLOCK)) as well. FIG. 7 demonstrates a possible relationship between the maximum switching frequency f_(CYC-MAX) and the output current I_(OUT). When the output current I_(OUT) exceeds a predetermined current I_(H), the output load 24 deems heavy and the maximum switching frequency f_(CYC-MAX) slowly varies within the range from 60 kHz to 66 kHz, with the jittering frequency of the control signal S_(JITTER). When the output current I_(OUT) is less than a predetermined current IL, the output load 24 deems light and the maximum switching frequency f_(CYC-MAX) slowly varies within the range from 25 kHz to 27.5 kHz, with the jittering frequency of the control signal S_(JITTER).

Shown in FIGS. 3 and 4, the ON time T_(ON) is in response to the compensation signal V_(COM), and the block time T_(BLOCK) in response to the load representative signal V_(L-EST).

As aforementioned, under a steady state when the output load 24 is a constant to make the output current I_(OUT) constant, the block time T_(BLOCK) could be constant, independent to any variation to the compensation signal V_(COMP). It implies that the power switch 34 could be turned on in a fix signal valley, avoiding the instability of valley switching and the possible audible noise possibly occurring in the prior art.

Furthermore, as demonstrated in FIGS. 3 and 7, the block time T_(BLOCK) is determined only by the output current I_(OUT) and the control signal S_(JITTER). It is well known that EMI measurement takes place only when the output current I_(OUT) is constant. Therefore, in some embodiments of the invention, during EMI measurement, the control signal S_(JITTER) could faithfully and slightly alter the block time T_(BLOCK), so as to jitter the switching frequency f_(CYC) and solve possible EMI issues.

The embodiments aforementioned so far are all QR converters, but the invention is not limited to, however. FIG. 8 shows a power controller 200, which in some embodiments of the invention replaces the QR controller 26 in FIG. 1 and does not operate the power switch 34 of FIG. 1 in QR mode. The power controller 200 of FIG. 8 has no valley detector 82 and the And gate 88 (of FIG. 3), and the inversion of the block signal S_(BLOCK) goes directly to the set terminal of the PWM 94. At the moment when the block time T_(BLOCK) ends, the PWM 94 is set, and the cycle time T_(CYC) and the ON time T_(ON) for the next switching cycle start. In other words, under the control of the power controller 200, the cycle time T_(CYC) is about the block time T_(BLOCK).

An embodiment of the invention substantially operates in QR mode, but, the transition of valley switching from one signal valley to another is not abrupt but “soft”. For example, a power converter according to the invention could perform valley switching in a 3^(rd) signal valley continuously, meaning a power switch is turned on when the 3^(rd) signal valley occurs. Then, possibly due to the increment to the output load, the time point when the power switch is turned on moves step-by-step from the moment when the 3^(rd) signal valley occurs to the moment when the 2^(nd) signal valley occurs. After several consecutive switching cycles, the power switch is turned on right at the moment when the 2^(nd) signal valley occurs, performing valley switching in the 2^(nd) signal valley. This transition process is referred to as soft transition for valley switching, which introduces one or more switching cycles not performing valley switching between two switching cycles performing valley switching in different signal valleys respectively.

FIG. 9 shows a QR controller 300, which in an embodiment of the invention is a replacement for the QR controller 26 in FIG. 1. FIGS. 9 and 3 have several apparatuses in common, and the similarity therebetween is comprehensible based upon the aforementioned teaching, so the similarity is not detailed due to brevity. The QR controller 300 has an OFF time controller 302 replacing the And gate 88 in the QR controller 80 (of FIG. 3). Most of time, the OFF time controller 302 performs valley switching, ending an OFF time T_(OFF) when the 1^(st) signal valley occurs after the conclusion of the block time T_(BLOCK). Nevertheless, under some circumstances, the OFF time controller 302 causes no valley switching, which will be detailed later.

FIG. 10 demonstrates some waveforms of signals in FIG. 1 when the QR controller 26 is replaced by the QR controller 300. Some waveforms in FIG. 10 have been shown in FIG. 4 and they are not explained redundantly.

An oscillation time T_(S-VL) is defined to refer to the duration beginning at a certain moment after the discharge time T_(DIS) ends and ending at the same moment when an OFF time T_(OFF) ends. The oscillation time T_(S-VL) shown in FIG. 10 starts at time point t₂ (when the discharge time T_(DIS) just ends) and ends at the time point t_(END) (when an OFF time T_(OFF) and a cycle time T_(CYC) conclude). In another embodiment, an oscillation time T_(S-VL) could be from time point t₃ (when the voltage drop V_(AUX) falls across 0V) or t₄ (when the first pulse in the valley indication signal S_(VD) appears) to the time point t_(END). The starting moment of the oscillation time T_(S-VL) is preferably selected to be no later than time point t₄ in FIG. 10, which is the moment when the first pulse in the valley indication signal S_(VD), after the end of the discharge time T_(DIS), appears. The oscillation time T_(S-VL) seems like, in a way, the total duration that the voltage drop V_(AUX) has been oscillating before the cycle time T_(CYC) or the OFF time T_(OFF) goes to end.

A prior oscillation time PT_(S-VL) is in association with the oscillation time T_(S-VL) in one of previous switching cycles that happened before. For example, the present oscillation time T_(S-VL) in the very present switching cycle could be the prior oscillation time PT_(S-VL) in the following switching cycle.

A time window TW is defined to be the duration between time points t_(W-S) and t_(W-E), both in response to the prior oscillation time PT_(S-VL). The time point t_(W-S) is the moment a predetermined lead period ahead when the prior oscillation time PT_(S-VL) concludes, while the time point t_(W-E) the moment a predetermined lag period behind when the prior oscillation time PT_(S-VL) concludes. It can be understood that, if a switching cycle lasts long enough, the moment when the prior oscillation time PT_(S-VL) ends is between time points t_(W-S) and time t_(W-E). The lead period and the lag period might be the same or different, and each is smaller than one oscillation cycle time T_(AUX-CYC) of the drop voltage V_(AUX), which is about the duration between two bottoms of two consecutive signal valleys. The oscillation cycle time T_(AUX-CYC) is also equal to the period between two consecutive moments when the drop voltage V_(AUX) falls across 0V, as shown in FIG. 10. Preferably, the length of the time window TW is less than one oscillation cycle time T_(AUX-CYC).

The time point t_(AB-1ST) refers to the moment when the 1^(st) pulse in the valley indication signal S_(VD) appears after time point t_(RELEASE). In other words, the time t_(AB-1st) is the moment when the 1^(st) signal valley occurs after the block time T_(BLOCK). It is unnecessary that the time point t_(AB-1ST) and the time point t_(END) are simultaneous as demonstrated in FIG. 10. In other words, the next switching cycle is not required to start at the time point t_(AB-1ST).

FIG. 11 shows a control method adapted by the OFF time controller 302 in FIG. 9. The OFF time controller 302 has a register to record a lock signal S_(LOCK). A lock signal S_(LOCK) with “1” in logic means the activation of valley locking, forcing that the valley switching for the present switching cycle should be performed in the same signal valley as it was done for the previous switching cycle. In the opposite, a lock signal S_(LOCK) with “0” in logic means the inactivation of valley locking, meaning that the present switching cycle is not required to perform valley switching in the same signal valley as before.

The OFF time controller 302 further records an oscillation time record RT, which is capable of providing the prior oscillation time PT_(S-VL) used in the present switching cycle. Step 306 provides the time window TW based on the prior oscillation time PT_(S-VL). In other words, step 306 determines time points t_(W-S) and t_(W-E), the beginning and ending of the time window TW respectively, based on the oscillation time record RT. As will be detailed later, it is not necessary that both time points t_(W-S) and t_(W-E) occur in a switching cycle. For example, the time point t_(W-E) might not happen because the present cycle time T_(CYC) concludes at the time point t_(W-S).

If the lock signal S_(LOCK) is “0”, meaning the inactivation of valley locking, step 308 has the time point t_(END) occur only within the time window TW. In step 308, the time point t_(END) is forbidden to appear earlier than the time point t_(W-S) or later than the time point t_(W-E). As to the exact moment of the occurrence of the time point t_(END), it depends on when the time point t_(AB-1ST) happens. If the time point t_(AB-1ST) happens ahead of the time t_(W-S), then the time point t_(END) is about simultaneous to the time point t_(W-S). Similarly, if the time point t_(W-E) happens while the time point t_(AB-1ST) has not happened, then the time point t_(END) is about simultaneous to the time point t_(W-E). Otherwise, if the time point t_(AB-1ST) happens within the time window TW, then the time point t_(END) is about simultaneous to the time t_(AB-1ST). According to aforementioned teaching, at time point t_(END), the PWM signal V_(GATE) has a rising edge to conclude both the cycle time T_(CYC) and the OFF time T_(OFF). The oscillation time record RT, after the conclusion of the OFF time T_(OFF), is updated using the present oscillation time T_(S-VL), to provide the prior oscillation time PT_(S-VL) used in the next switching cycle. For the present embodiment, the moment when the OFF time T_(OFF) concludes is in response to the time window TW and the time point t_(AB-1ST), while the time window TW is determined by the oscillation time record RT, and the time point t_(AB-1ST) is determined by the block time T_(BLOCK) and the valley indication signal S_(VD).

If the lock signal S_(LOCK) is “1”, meaning valley locking is expected, step 316 has the time point t_(END) occur about at the same time when the prior oscillation time PT_(S-VL) concludes. The oscillation time T_(S-VL) for the present switching cycle will be the same with that for the previous switching cycle. If the previous switching cycle performs valley switching in a specific signal valley, then the present switching cycle will also perform valley switching right in the very specific signal valley. It seems like that valley switching is locked to perform constantly in the specific signal valley if the lock signal S_(LOCK) is “1”. That explains the terminology of valley locking.

The OFF time controller 302 in FIG. 9 further has a counter for counting how many switching cycles the valley locking has been performed, as shown in step 320 in FIG. 11. The counter also seems like a timer to calculate the duration when the valley locking has lasted. Step 322 demonstrates that the lock signal S_(LOCK) is reset to “0” from “1” to disable or inactivate the valley locking if the count of the counter reaches a predetermined number N. In other words, the lock signal S_(LOCK) with “1” must last for N consecutive switching cycles before being reset. After the valley locking is disabled or inactivated, if step 310 determines that the time point t_(AB-1ST) does not happen within the time window TW, the present switching cycle is not performing valley switching, such that step 315 resets the counter to have the count be 0. Once the time point t_(AB-1ST) reenters the time window TW as being determined by step 310, the present switching cycle starts performing valley switching, such that step 314 sets the lock signal S_(LOCK) “1” in logic and increases the count by 1.

Please reference FIGS. 1, 9, 11 and 12 for the following, where FIG. 12 shows the waveforms of the drop voltage V_(AUX) and several signal timings during several consecutive switching cycles when the output load 24 turns from heavy into light.

It is assumed that the X^(th) switching cycle in FIG. 12 has reached a stable condition, where the OFF time controller 302 renders valley switching substantially at the bottom of the signal valley VL₂. During the X^(th) switching cycle, the time point t_(AB-1ST) is also the time point t_(END), which is the end of a cycle time T_(CYC), the oscillation time T_(S-VL) is the same with the prior oscillation time PT_(S-VL), the lock signal S_(LOCK) is “0” in logic, and the count of the counter is N. As the time window TW has not completed before the OFF time T_(OFF) ends, the time point t_(W-E) actually does not occur even though it is illustratively shown there for reference. The OFF time T_(OFF) for the X^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 312 and 324.

In the beginning of the (X+1)^(th) switching cycle, probably because the output load becomes lighter suddenly, the output current I_(OUT) decreases, the block time T_(BLOCK) becomes longer and the time point t_(RELEASE) is lagged, such that the time point t_(AB-1ST) has not occurred when the time window TW completes in the (X+1)^(th) switching cycle. The OFF time T_(OFF) for the (X+1)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 315 and 324. As demonstrated in FIG. 12, for the (X+1)^(TH) switching cycle, the time point t_(END) is about the same with the time point t_(W-E), the lock signal is “0” in logic, and the count is 0. The oscillation time T_(S-VL) is the lag period more than the prior oscillation time PT_(S-VL) as demonstrated in FIG. 12, and this lag period is only a portion of one oscillation cycle time T_(AUX-CYC) of the drop voltage V_(AUX). In FIG. 12, this lag period is less than half oscillation cycle time T_(AUX-CYC) of the drop voltage V_(AUX). Accordingly, it is obvious in FIG. 12 that the (X+1)^(th) switching cycle does not perform valley switching.

During the (X+2)^(th) switching cycle in FIG. 12, the time point t_(AB-1ST) is still absent when the time window TW is over. Similar with what happened in the (X+1)^(th) switching cycle, the OFF time T_(OFF) for the (X+2)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 315 and 324. The time point t_(END) is about the same with the time point t_(W-E), the lock signal is “0” in logic, and the count is 0. The (X+2)^(th) switching cycle does not perform valley switching, either.

During the (X+3)^(th) switching cycle in FIG. 12, the time point t_(AB-1ST) appears inside the time window TW. Accordingly, the OFF time T_(OFF) for the (X+3)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 312 and 314. As shown in FIG. 12, the time point t_(END) is about the same with the time point t_(AB-1ST), the lock signal becomes “1” in logic, and the count now is 1. Unlike the (X+2)^(TH) switching cycle, the (X+3)^(TH) switching cycle performs valley switching, and the valley locking is activated from now on.

Because the valley locking has been activated at the beginning of the (X+4)^(th) switching cycle, the time t_(END) of the (X+4)^(th) switching cycle is forced to be about the end of the prior oscillation time PT_(S-VL). The OFF time T_(OFF) for the (X+4)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 316, 318, and 320. The prior oscillation time PT_(S-VL) is not updated even though the oscillation time T_(S-VL) is the same with the prior oscillation time PT_(S-VL). The lock signal is still “1” in logic, and the count now becomes 2. The (X+4)^(th) switching cycle performs valley switching, too.

As shown from the process going from the X^(th) switching cycle to the (X+4)^(th) switching cycle, the oscillation time T_(S-VL) increases cycle-by-cycle. The end of the oscillation time T_(S-VL) starts first at the bottom of the signal valley VL₂, shifts a little bit later cycle-by-cycle, and stays finally at the bottom of the signal valley VL₃, as demonstrated in FIG. 12. The OFF time controller 302 limits the difference between the prior oscillation time PT_(S-VL) and the oscillation time T_(S-VL) to be less than one oscillation cycle time T_(AUX-CYC).

After the (X+4)^(th) switching cycle, both the prior oscillation time PT_(S-VL) and the oscillation time T_(S-VL) stay unchanged and about equal to each other if the lock signal S_(LOCK) is “1”. The OFF time T_(OFF) for the following switching cycles could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 316, 318, and 320. As shown in FIG. 12, the count increases by 1 for each switching cycle, and the lock signal S_(LOCK) stays as “1”. Eventually, when the count increases to N, the lock signal S_(LOCK) will turn to “0” to inactivate the valley locking.

Please reference FIGS. 1, 9, 11 and 13 for the following, where FIG. 13 shows the waveforms of the drop voltage V_(AUX) and several signal timings during several consecutive switching cycles when an output load turns from light into heavy.

It is assumed that the Y^(th) switching cycle has reached a stable condition, where the OFF time controller 302 performs valley switching substantially at the bottom of the signal valley VL₃, similar to what happens in the final switching cycle in FIG. 12. During the Y^(th) switching cycle, the time point t_(AB-1ST) is about the time point t_(END), which is the end of a cycle time T_(CYC), the oscillation time T_(S-VL) is the same with the prior oscillation time PT_(S-VL), the lock signal S_(LOCK) is “0” in logic, and the count of the counter is N. The OFF time T_(OFF) for the Y^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 312 and 324.

In the beginning of the (Y+1)^(th) switching cycle, probably because the output load becomes heavier suddenly, the time point t_(RELEASE) is leaded to occur around the end of the signal valley VL₁, such that the time point t_(AB-1ST) occurs earlier than the moment when the time window TW starts. The OFF time T_(OFF) for the (Y+1)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 315 and 324. As demonstrated in FIG. 13, for the (Y+1)^(th) switching cycle, the time point t_(END) is about the same with the time point t_(W-S), the lock signal S_(LOCK) is “0” in logic, and the count is 0. The oscillation time T_(S-VL) is a lead period shorter than the prior oscillation time PT_(S-VL) as demonstrated in FIG. 13, and this lead period is only a portion of one oscillation cycle time of the drop voltage V_(AUX). In FIG. 13, this lead period is less than half oscillation cycle time of the drop voltage V_(AUX). Accordingly, it is obvious in FIG. 13 that the (Y+1)^(th) switching cycle does not perform valley switching.

During the (Y+2)^(th) switching cycle in FIG. 13, the time point t_(AB-1ST) still occurs prior to the beginning of the time window TW. Accordingly, the OFF time T_(OFF) for the (Y+2)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 315 and 324. The time point t_(END) is still about the same with the time point t_(W-S), the lock signal is “0” in logic, and the count is 0. The (Y+2)^(th) switching cycle does not perform valley switching, either.

During the (Y+3)^(th) switching cycle in FIG. 13, the time point t_(AB-1ST) appears inside the time window TW. Accordingly, the OFF time T_(OFF) for the (Y+3)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 306, 308, 310, 312 and 314. As shown in FIG. 13, the time point t_(END) is about the same with the time point t_(AB-1ST), the lock signal becomes “1” in logic, and the count now is 1. Unlike the (Y+2)^(th) switching cycle, the (Y+3)^(TH) switching cycle performs valley switching, and the valley locking is activated from now on.

Because the valley locking has been activated before the beginning of the (Y+4)^(th) switching cycle, the time point t_(END) is forced to be about the end of the prior oscillation time PT_(S-VL). The OFF time T_(OFF) for the (Y+4)^(th) switching cycle could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 316, 318, and 320. The prior oscillation time PT_(S-VL) is not updated even though the oscillation time T_(S-VL) is the same with the prior oscillation time PT_(S-VL). The lock signal S_(LOCK) is still “1” in logic, and the count now becomes 2. The (Y+4)^(th) switching cycle performs valley switching, too.

As shown from the process going from the Y^(th) switching cycle to the (Y+4)^(th) switching cycle, the oscillation time T_(S-VL) decreases cycle-by-cycle. The end of the oscillation time T_(S-VL) starts first at the bottom of the signal valley VL₃, shifts a little bit earlier cycle-by-cycle, and stays finally at the bottom of the signal valley VL₂, as demonstrated in FIG. 13.

After the (Y+4)^(th) switching cycle, both the prior oscillation time PT_(S-VL) and the oscillation time T_(S-VL) stay unchanged and about equal to each other. The OFF time T_(OFF) for the following switching cycles could be derived from FIG. 11, based on the flow consisting of the steps 304, 305, 316, 318, and 320. As shown in FIG. 13, the count increases by 1 for each switching cycle, and the lock signal S_(LOCK) stays as “1”. Eventually, when the count increases to N, the lock signal S_(LOCK) will turn to “0” to disable or inactivate the valley locking.

The teaching of FIGS. 11, 12 and 13 exemplifies that the valley locking to a certain signal valley is activated the first time when valley switching in the certain signal valley is performed, and that the valley locking is disabled or inactivated after the valley switching in the certain signal valley has continued for N consecutive switching cycles. Soft transition for valley switching is also exemplified in FIGS. 11, 12, and 13, where at least one switching cycle not performing valley switching is inserted between two switching cycles performing valley switching in two neighboring signal valleys, respectively.

FIG. 14 shows possible variation to the oscillation time T_(S-VL) of the prior art, which performs neither soft transition for valley switching, nor valley locking. Demonstrated in FIG. 14, because of the lack of soft transition for valley switching, the difference between two oscillation times T_(S-VL) of two different switching cycles must be an integral number of the oscillation cycle time T_(AUX-CYC). As aforementioned, the oscillation cycle time T_(AUX-CYC) is about equal to the time difference between two neighboring bottoms of signal valleys. As the oscillation times T_(S-VL) might change largely to an extent of several oscillation cycle times T_(AUX-CYC), the power converter in the prior art might be unstable and has large output ripple in the output voltage V_(OUT).

Furthermore, the prior art in FIG. 14 lacks the technique of valley locking, such that the valley switching might jump back and forth quickly in two neighboring signal valleys, as shown in FIG. 14.

FIG. 15 shows possible variation to the oscillation time T_(S-VL) according to one embodiment of the invention. FIG. 15 demonstrates the result of the soft transition for valley switching, as the valley switching in signal valley VL₄ transits to the valley switching in signal valley VL₃ softly via three consecutive switching cycles performing no valley switching. FIG. 15 also demonstrates the result of the valley locking, where the valley switching in signal valley VL₃ is performed at least 8 times (for 8 consecutive switching cycles) before transiting to the valley switching of a neighboring signal valley. By comparing with the oscillation times T_(S-VL) in FIG. 14, the oscillation times T_(S-VL) in FIG. 15 varies smoother, making a power converter much more stable.

The QR controller 300 of FIG. 9 introduces 3 different techniques. One is the block time T_(BLOCK) in response to the load representative signal V_(L-EST); another is the soft transition for valley switching; and the other is the valley locking. This invention is not limited to what is introduced in FIG. 9, nevertheless. One embodiment of the invention might perform only one of the three techniques, any two of the three techniques, or all of the three techniques. For example, one embodiment of the invention has the functions of both the block time T_(BLOCK) in response to the load representative signal V_(L-EST) and the soft transition for valley switching, but lacks the function of the valley locking. Another embodiment might be able to perform the soft transition for valley switching and the valley locking, but its block time T_(BLOCK) is in response to the compensation signal V_(COMP) rather than the load representative signal V_(L-EST).

While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A power controller for a switching mode power supply, wherein the switching mode power supply has a power switch connected in series with an inductive device, a voltage drop of the inductive device is capable of oscillating to generate at least one signal valley, and the power switch is controlled by a PWM signal defining an ON time and an OFF time, the power controller comprising: a valley detector coupled to the inductive device, for providing a valley indication signal indicating the occurrence of the signal valley; a block time generator for providing a block time; and an OFF time controller, for recording an oscillation time record to provide a prior oscillation time, for concluding the OFF time in response to the oscillation time record, the block time, and the valley indication signal, and for updating the oscillation time record basing on a present oscillation time, wherein the prior oscillation time is in association with a previous switching cycle; wherein the present oscillation time starts after the voltage drop has oscillated and ends when the OFF time is concluded.
 2. The power controller as claimed in claim 1, wherein the OFF time controller is capable of providing a time window in response to the oscillation time record, and the prior oscillation time ends within the time window.
 3. The power controller as claimed in claim 2, wherein the OFF time controller forces the OFF time to conclude within the time window.
 4. The power controller as claimed in claim 2, wherein: after the block time ends, the valley indication signal indicates the occurrence of the first signal valley at a time point; if the time point is ahead of the time window, the OFF time concludes at the beginning of the time window; if the time point is behind the time window, the OFF time concludes at the ending of the time window; and if the time point is within the time window, the OFF time concludes at the time point.
 5. The power controller as claimed in claim 2, the time window has a length less than an oscillation cycle time of the voltage drop.
 6. The power controller as claimed in claim 1, wherein the OFF time controller limits the difference between the prior oscillation time and the prior oscillation time to be less than one oscillation cycle time of the voltage drop.
 7. The power controller as claimed in claim 6, wherein the OFF time controller has a counter for counting how many consecutive switching cycles have performing valley switching.
 8. A control method for a switching mode power supply with an inductive device and a power switch, wherein the power switch is controlled by a PWM signal capable of defining a switching cycle consisting of an ON time and an OFF time, a voltage drop of the inductive device is capable of oscillating to have an oscillation cycle time and generate at least one signal valley, the control method comprising: providing an oscillation time record to represent a prior oscillation time in association with a previous switching cycle; during a present switching cycle, controlling the PWM signal to conclude the OFF time in response to the prior oscillation time; and updating the oscillation time record based on a present oscillation time; wherein the present oscillation time starts after the voltage drop has oscillated and ends when the OFF time is concluded; the difference between the present oscillation time and the prior oscillation time is less than an oscillation cycle time of the voltage drop, wherein a first switching cycle of the previous switching cycle and the present switching cycle performs valley switching, and a second switching cycle of the previous switching cycle and the present switching cycle does not perform valley switching.
 9. The control method as claimed in claim 8, further comprising: providing a time window based on the prior oscillation time record; limiting the OFF time to conclude within the time window.
 10. The control method as claimed in claim 9, further comprising: after a block time ends, indicating the occurrence of the first signal valley at a time point; if the time point is ahead of the time window, concluding the OFF time at the beginning of the time window; if the time point is behind the time window, concluding the OFF time at the ending of the time window; and if the time point is within the time window, concluding the OFF time at the time point.
 11. The control method as claimed in claim 8, further comprising: activating valley locking to conclude the OFF time and the prior oscillation time at the same time; and inactivating the valley locking after a predetermined number of switching cycles has performed the valley locking.
 12. A control method for a switching mode power supply with a power switch and an inductive device connected in series, wherein a voltage drop of the inductive device is capable of oscillating to have an oscillation cycle time and to generate at least one signal valley, and a present oscillation time in a switching cycle is a duration starting after the voltage drop has oscillated and ending when the switching cycle concludes, the control method comprising: for a first switching cycle, providing a PWM signal to control the power switch and generate a first oscillation time, so as to perform valley switching; for a second switching cycle behind the first switching cycle, providing the PWM signal to control the power switch and generate a second oscillation time, so as not to perform valley switching; and for a third switching cycle behind the second switching cycle, providing the PWM signal to control the power switch and generate a third oscillation time, so as to perform valley switching; wherein the difference between the first and second oscillation times is less than one oscillation cycle time; and the difference between the second and third oscillation times is less than one oscillation cycle time.
 13. The control method as claimed in claim 12, wherein the first oscillation time is different from the third oscillation time. 